Joining (Ctrl + J) multiple paths bevelled by a shape ruins the whole shape
This issue is most likely caused by modifiers either having different settings or operating differently because of a sudden change in relativity to the origin point since upon joining the objects it uses the origin from the selected object, If this is the case I would suggest ensuring all the origins are in the same location before joining them. If not make sure there are no differences in modifier settings before merging them or simply apply the modifiers before merging the objects together as user DontWalk commented